Metal matrix composites are formed when graphite fibers are embodied in light metals such as aluminum, magnesium, and titanium. These composites are used in structures which are subject to severe environments since they have low mass density, low thermal explansion, high strength, and high thermal and electricdal conductivities.
Metal matrix composites are conventionally produced by infiltrating tows of graphite fibers with molten metal to produce precursor wires which are subsequently collimated and consolidated to form the composite. However, this technique produces material with low transverse tensile strength and cannot produce sheets less than one-half millimeter thick.
To overcome these deficiencies, a method for the physical vapor deposition of metal matrix onto graphite fibers has been developed. This produces ultra-thin composite precursor tapes. Multiple layers of these tapes are consolidated by a hot diffusion bonding process to form composite sheets as thin as 0.1 millimeter. A uniform coating of metal onto individual filaments of these procursor tapes is required for their successful consolidation. When the fibers are not well spread, the interior fibers receive an insufficient coating of metal during the physical vapor deposition process. The absence of coating on the interior fibers, and the resulting non-uniform metal matrix distribution in the consolidated end product, produce a metal matrix composite with inferior strength properties.
A previous attempt at pneumatic spreading of filaments used compressed air jets through pipes to fluff or lay open bundles of fibrous strands while they were collimated and spread. U.S. Pat. No. 3,873,389, Pneumatic Spreading of Filaments, Clare G. Daniels, Mar. 25, 1975. Restrictors were used to adjust the width of the openings of the pipes. However, the restrictors were installed normal to the pipe width such that the fibers were forced to spread as a result of the restriction of pipe dimension rather than as a direct result of the Venturi effect. Tows were spread to a compact tape form of several layers thick without any control over the spacing between adjacent collimated fibers.
We previously attempted to develop a fiber spreading technique using a mechanical method wherein the fiber tow was pulled through a series of laterally oscillating rollers. The term "fiber" used collectively, and the term "fibers," are used interchangeably herein to denote a collection of strands which, collectively, constitute fiber tows. The alternate motion of the rollers spread the fiber tow by frictional force into a compact tape of two or three fibers thick with no control over the spacings between the fibers. Furthermore, fiber breakage due to the frictional nature of mechanical spreading presented process complications.